The invention relates to a method for reducing the electrical resistance on the current-carrying transition to a component made of chromium steel due to a corrosion-caused surface layer of oxide on the component. It also has applications of this method as its subject.
In many technical apparatus chromium steel is used for many kinds of components for reasons of strength and corrosion resistance. This material resists corrosion attack by forming a protective chromium oxide coating. The chromium oxide coating is a good barrier against diffusion and thus prevents corrosion attack. Especially in high-temperature applications the chromium oxide coating leads to a great reduction of oxide growth and thus to a lasting protection against the destruction of the material.
If chromium steel, however, is used for electric current carrying components in high-temperature applications in an oxidizing atmosphere, the necessary formation of the chromium oxide coating causes an electrical resistance caused by this coating, which in turn leads to voltage losses and thus to a lowering of the efficiency of the apparatus in question.
An example of a component which is exposed to an oxidizing atmosphere at high temperatures is the cathode current collector of a molten carbonate fuel cell (MCFC).
Molten carbonate fuel cells consist essentially of a porous cathode and a porous anode and a matrix which is imbibed with a molten electrolyte, namely a eutectic mixture containing a lithium carbonate and other alkali carbonates which is in contact with the electrodes. To carry the electrochemically produced current, current collectors are in contact with the cathode and the anode, and they are usually corrugated in order also to form a gas transport space, and namely for carrying air or other oxygen-containing gas at the cathode and a fuel gas at the anode. The fuel cell is operated at a temperature of 500 to 800.degree. C. The cathode current collector is exposed to severe corrosive influences by contact with the oxygen-containing gas and the molten carbonate as well as by the high temperature. In spite of the use of chromium steel, an oxide coating thus forms on the surface of the cathode current collector, which leads to a high resistance to transition between the cathode current collector and the cathode and thus to high power losses in the molten carbonate fuel cell.
To lower this transition resistance, it is proposed according to DE 195 32 791 A1 to apply a noble metal, such as gold or platinum, to the cathode current collector at the points of contact with the cathode. Aside from the high costs of the noble metals, this method has the disadvantage that a diffusion barrier must still be placed between the thin noble metal coating and the chromium steel cathode current collector in order to prevent the diffusion of the noble metal into the chromium steel. Thus several coating steps are necessary.
The invention is addressed to the problem of offering a method whereby the electrical resistance at the transition to a current-carrying chromium steel component due to a surface oxide coating caused by corrosion on the component can be reduced in a simple manner.
This is achieved according to the invention by annealing the chromium steel component a temperature of at least 950.degree. C., and removing any oxide coating that develops during said annealing before the component carries current.